1. Field of the Invention
The present invention relates to an ABA triblock copolymer, a thickener, and an aqueous composition.
2. Description of the Related Art
It has been expected that a reversible gelling material, which induces a sol-gel change of a system, such as a composition containing a medium (e.g. water) due to reversible viscosity increase or degrease, is applied, as one type of viscosity controlling agents, such as a thickener, for drug carriers for a drug delivery system or the like, regenerative medicine materials for tissue engineering or the like, actuators or pumps for artificial muscle, cosmetic materials, display elements, recording elements, energy elements of solar batteries or the like, additives for inks, coating agents for paper for certain use, and various functional films.
As for a factor for giving the reversible viscosity change to the system, for example, included are temperature, pH (strictly speaking, H+), light, electric field, ion, and addition of a solvent. Among them, pH has been attracted attention as the factor for giving reversibility, because a reversible change can be promptly given without using a complicated device, and there are portions where pH thereof are different within organisms.
In the case where the factor for giving reversibility is pH, a system containing a medium induces a viscosity change, such as a sol-gel change, due to a difference between an associated state, and a non-associated state at around an acid dissociation constant (pKa).
Polyacrylic acid, which is a pH-responsive polymer induces a reversible viscosity change to a system due to a difference in pH, is a polymer having carboxylic acid pKa of which is 5 to 6. The polyacrylic acid receives protons at low pH, but dissociates protons at high pH, so that it tends to be easily dissolved due to electrostatic repulsion. For example, in the case where a system containing crosslinked polyacrylic acid, which is chemically crosslinked, is acidic, a carboxyl group of the polyacrylic acid is in the non-dissociated state, and therefore it is contracted. Accordingly, such system has low viscosity. In the case where a system containing the crosslinked polyacrylic acid is alkaline, on the other hand, the crosslinked polyacrylic acid is swollen due to electrostatic repulsion of dissociated carboxyl ions to thereby form a microgel. Accordingly, such system has high viscosity. Meanwhile, chitosan, crosslinked poly-N-dimethylaminoethyl methacrylate (PDMAEMA), and crosslinked poly-N-diethylaminoethyl methacrylate (PDEAEMA) each contain an amino group in a molecule thereof. In the case where a system containing any of these polymers is acidic, the system has high viscosity due to electrostatic repulsion of ammonium ions.
The pH-responsive polymer exhibiting a thickening effect due to cubical expansion caused by electrostatic repulsion, such as the crosslinked polyacrylic acid, and the crosslinked poly-N-diethylaminoethyl methacrylate, are however influenced by ions present in a system. In the case where a system contains many ions, such as an energy element contains an electrolyte or a dye ink, there is a problem that a thickening effect is hardly attained. In the case of the pH-responsive polymer utilizing cubical expansion, moreover, there are problems that it takes time to cause a volumetric change, and a response time of a thickening effect is slow.
As a pH-responsive polymer that solves the aforementioned problems, there is a polymer, which induces a viscosity change to a system utilizing hydrophobic association. As a polymer that induces a viscosity change to a system utilizing hydrophobic association, for example, proposed is an ABA triblock copolymer having 2-diisopropylaminoethyl methacrylate (DPA) or 2-diethylaminoethyl methacrylate (DEA) as an A-block, and 2-methacryloyloxyethylphosphorylcholine (MPC) as a B-block (see, for example, Biomacromolecules, Vol. 4, No. 4, 2003 864-868). The ABA triblock copolymer of the disclosed technology is a polymer that exhibits a thickening effect even when an electrolyte is added to a system.
In order for the ABA triblock copolymer of the disclosed technology to exhibit a sufficient thickening effect, for example, to gelatinize a system, it is necessary to contain 40 mol % or more of a pH responsive monomer (DPA, DEA) per molecule of the polymer, and moreover, about 10% by mass of the polymer is required in water. Therefore, there are problems that a significant amount of alkali is required to deprotonate ammonium ions, and a response speed required for sufficient gelation becomes slow.
Moreover, the polymer induces a viscosity change to a system using hydrophobic association typically requires a high polymer concentration in the system to exhibit a sufficient thickening effect, and there fore there is a problem that a material cost is high, and the viscosity of the system is high even before exhibiting a reversible thickening effect.
Note that, the above disclosed technology is an example of a material that induces a viscosity change to a system utilizing hydrophobic association under an alkaline condition. As for a material for hydrophobing under an acidic condition and an application example thereof, disclosed are a diblock copolymer (MAU/AMPS=95/5 (molar ratio)) of a monomer that becomes hydrophobic (e.g., 11-methacryl amide undecanoic acid (MAU)) under an acidic condition, and a hydrophilic monomer regardless of pH (e.g., 2-acryl amide-2-methylpropane sulfonic acid (AMPS)), and a cosmetic composition containing such polymer as a powder surface treating agent (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-210903). The disclosed technology has a function as a dispersing agent for improving dispersibility of a powder material, such as titanium oxide, a function for giving water repellency as it is applied on skin, and washability with alkali, such as soap. The polymer used in this technique is a polymer material utilizing a reversible change between hydrophilic and hydrophobic natures, but has hardly any function for inducing a reversible viscosity change.
Accordingly, it is currently desired to provide a pH responsive polymer, which can be used in a system where an electrolyte is present, has a short response time to a viscosity change, can exhibits a reversible thickening effect with a low concentration thereof.